Terephthalic acid and other aromatic carboxylic acids are widely used in the manufacture of polyesters, commonly by reaction with one or more glycols, and particularly ethylene glycol and combinations thereof with one or more higher homologues of alkylene glycols, for conversion to fiber, film, containers, bottles and other packaging materials, and molded articles.
In commercial practice, aromatic carboxylic acids are commonly made by liquid phase oxidation in an aqueous acetic acid solvent of methyl-substituted benzene and naphthalene feedstocks, in which the positions of the methyl substituents correspond to the positions of carboxyl groups in the desired aromatic carboxylic acid product. Oxidation is conducted by contacting the feedstock with air or another source of oxygen, which is normally gaseous, in the presence of a catalyst comprising cobalt and manganese promoted with a source of reactive bromine. The oxidation is exothermic and yields aromatic carboxylic acid together with by-products, including partial or intermediate oxidation products of the aromatic feedstock as well as oxidation and other reaction products of the acetic acid solvent such as methanol, methyl acetate, methyl bromide, carbon monoxide and carbon dioxide. Water is also generated as a by-product. The aromatic carboxylic acid oxidation product, by-products and intermediate oxidation products of the feedstock are commonly formed dissolved or as solids suspended in the liquid phase reaction mixture and are recovered by crystallization and solid-liquid separation techniques.
Pure forms of aromatic carboxylic acids are often favored for manufacture of polyesters for important applications, such as fibers, bottles, and other containers and packaging materials, because impurities, such as by-products generated from aromatic feedstocks, cause or correlate with color formation in polyesters made from the carboxylic acids and, in turn, off-color in polyester converted products. Aromatic carboxylic acids with reduced levels of impurities can be made by further oxidizing crude products from liquid phase oxidation, for example at one or more progressively lower temperatures and/or oxygen levels or during crystallization steps used to recover products of the oxidation, to convert partial oxidation products to the desired acid product. Preferred pure forms of terephthalic acid and other aromatic carboxylic acids with lower impurities contents are made by catalytically hydrogenating less pure forms of the acids in solution at elevated temperature and pressure using a noble metal catalyst. In commercial operations, liquid phase oxidation of alkyl aromatic feed materials to crude aromatic carboxylic acid and purification of the crude product are often conducted in continuous integrated processes in which a starting material for purification comprises crude product from oxidation.
A difficulty in manufacture of aromatic carboxylic acids results from use of bromine-promoted oxidation catalysts. Bromine sources used with the catalyst and reaction products thereof formed during oxidation are corrosive. Consequently, process equipment, such as oxidation reactors and off-gas treatment equipment, is normally constructed from titanium or other expensive, corrosion-resistant metals or alloys. In addition, process off-gas treatments to avoid atmospheric emissions of volatile bromine compounds, such as thermal or catalytic oxidation to convert organic bromine compounds to carbon oxides and molecular bromine with reduction of the latter to anionic bromine using sodium formate, add complexity and cost to manufacturing processes.
Eliminating bromine from cobalt-manganese oxidation catalysts is not practical for commercial scale aromatic carboxylic acid manufacture because yields of desired products are unacceptably low. Oxidation of acetic acid reaction solvent also tends to increase in cobalt and manganese-catalyzed oxidations without bromine. Sacrificial promoters, such as methyl ethyl ketone and acetaldehyde, have been proposed as alternatives to bromine but they are consumed in oxidation, thereby adding costs for their replacement and diverting oxygen from desired reactions. Sacrificial promoters can also negatively affect product quality in higher temperature oxidations. N-hydroxyphthalamide has been reported as a bromine-free promoter for cobalt-catalyzed reactions but lacks practical utility due to low solubility in acetic acid and conversion by multiple competing decomposition reactions to undesirable by-products.
Noble metal-catalysts for oxidizing methylated benzenes are proposed in U.S. Pat. No. 3,865,870 but conversions and selectivities to aromatic carboxylic acids are low and carbon oxides generation is high. Selective oxidation of alkanes and alkenes to aldehydes and ketones using catalysts containing noble metal particles dispersed on antimony oxide and having 1-30 mole % of the particles in the form of a noble metal-antimony alloy is proposed in U.S. Pat. No. 5,864,051 but there is no mention that the catalysts have any use for making carboxylic acids of for oxidation of aromatic compounds. Acetoxylation of substituted aromatic compounds by reaction with aliphatic monocarboxylic acids and oxygen using noble metal catalysts can yield benzylic alcohols and their esters with the monocarboxylic acids but with conversion does not proceed beyond the benzylic alcohols to more fully oxidized derivatives.